Persistent connections
Web pages that consist of text alone are retrieved from a web server with one HTTP request. However, many web pages contain images and other objects that require additional HTTP requests; some corporate home pages contain more that 60 images, each requiring a separate HTTP request. The page shown in Example B-4 requires three requests: the first to get the page and then two more to get the images. While the browser is usually configured to make up to four parallel requests, with HTTP/1.0, each request must open a separate TCP connection to the server. This is expensive as each request adds to the cost of the TCP dialog required to open the virtual connection.
As a result, browsers and web servers often support a keep-alive capability to reduce the time taken to load web pages that contain images. This feature allows multiple requests to be made in succession using the one TCP connection. After a specified idle period, the connection is assumed to be no longer required and is closed by the server. HTTP/1.1 uses persistent TCP connections as the default behavior. Persistent connections not only improve the performance of the browser but reduce the load on a HTTP server and on the network, because fewer TCP connections need to be established.
In the same way as the simultaneous request model, a browser can send multiple requests on a persistent connection without waiting for each response. This behavior is called pipelining.